A liquid crystal display (LCD) useful as an optical display device is an indirect light-emitting device which shows an image by controlling the transmittance of an external light source, and the light source device thereof, a backlight unit, is an important part helping to determine the properties of the LCD.
In particular, as a technique for fabricating an LCD panel becomes more devolved, the demand for LCDs which are slim and have high luminance is increased. Accordingly, various attempts to increase the luminance of the backlight unit have been made. The LCD suitable for use in monitors, PDAs (Personal Digital Assistants), and notebook computers is evaluated to be superior when it emits bright light rays from a low energy source. Thus, in the case of the LCD, front-surface luminance is regarded as very important.
Because the LCD is configured such that light passing through a light diffusion layer is diffused in all directions, the quantity of light traveling toward the front surface is much too low. Hence, great effort is continually made into exhibiting higher luminance with lower power consumption. Further, as the area of the display is enlarged, the viewing angle is required to be wider to enable the image to be viewed by more observers.
To this end, the power of the backlight unit may be increased. However, power consumption and concomitant power loss attributable to heat are increased in proportion to the increase thereof. In the case of portable displays, the capacity of the battery must be increased, and the life span thereof is shortened.
Hence, with the goal of increasing the luminance, methods of imparting light with directional properties have been proposed. For this, various lens sheets have been developed. A representative example thereof is an optical sheet having a prism array on the surface thereof.
Typically, an optical sheet having a prism array is configured such that it has a triangular array having a surface tilted at 45° to improve front-surface luminance.
However, because the structured surface of the optical sheet has mountain-shaped prism structures, peaks of the prism structures may be easily broken or distorted by small scratches on the exterior, undesirably causing the problem of damaging the prism structures. Further, all the angles emitted from the same-shaped prism structures of the array are the same. In the case where the peaks of the structures in the prism array are slightly destroyed or small scratches are created in the tilted surface thereof, the emitted light path between the damaged portion and the normal portion becomes different, undesirably reducing luminance and increasing the rate of defection. So, in the production of a prism sheet, problems in which the front surface of the prism sheet cannot be used depending on the position of small defects may arise, undesirably leading to a decrease in productivity and thus to a high cost burden. Actually, manufacturers who assemble backlight modules considerably suffer because of the high defective rates attributable to damage to prism structures by scratches caused when the prism sheets are handled.
Moreover, in the backlight unit consisting of a plurality of sheets and films which are layered, a plurality of prism films may be provided to increase luminance. In this case, when the upper surface of the lower prism film comes into contact with the lower surface of the upper prism film, the prism structures may be disadvantageously damaged.
Thus, in order to prevent damage to the prism structures, use of a protective film has been proposed. However, as the LCD panel is required to be slimmer, the general trend is to omit such a film or to use a multifunctional sheet, and also, if a process of forming a protective film is added, the production cost is increased and temporal and physical efficiencies are decreased.
In addition to damage to the prism structures attributable to handling, while portable displays, such as notebook computers and PDAs, are increasingly used, they are frequently transported in a state of being placed in a bag. During the transport thereof, when impact is applied to the display when a user runs or a car stops suddenly, the prism structures in the display are damaged even in the presence of the protective film, negatively affecting the image screen.
Therefore, there is an urgent need for an optical sheet having a structured surface capable of flexibly accommodating external impact.
Typically, the backlight unit causes light to be emitted using a light source such as a cold cathode fluorescent lamp (CCFL), so that such emitted light is sequentially passed through a light guide plate, a light diffusion element such as a light diffusion sheet or a light diffusion plate, and a prism sheet, before reaching a liquid crystal panel. The light guide plate functions to transfer light emitted from the light source in such a manner as to distribute it over the front surface of the liquid crystal panel, which is planar, and the light diffusion element plays a role in realizing uniform light intensity over the front surface of a screen. The prism sheet functions to control the light path so that light traveling in various directions through the light diffusion element is transformed within a range of viewing angles θ suitable for enabling the image to be viewed by an observer. Further, a reflection sheet is provided under the light guide plate to reflect that light which does not reach the liquid crystal panel and is outside of the light path, so that such light is used again, thereby increasing the efficient use of the light source.
So, in order to effectively transfer the emitted light to the liquid crystal panel, a plurality of sheets having various functions is provided. As a result of the use of the plurality of sheets, however, light interference occurs, and also the films may become damaged owing to physical contact between the sheets, undesirably causing problems such as low productivity and high cost.
Recent attempts to reduce the number of optical sheets in order to simplify the production process have been made. Examples of such are where a prism film is adhered onto a light diffusion element or prism patterns are formed on a light diffusion element. Such a plate is advantageous in terms of cost or productivity, but is problematic in that an increase in luminance falls very short of expectations.
Therefore, methods of sufficiently increasing luminance while at the same time minimizing the use of optical sheets to increase it are required.